1. Field of the Invention
The present invention concerns a processing solution tank of forming an agitating flow by flowing and circulating a processing solution in directions opposite to each other between the side of a liquid surface and the side of a tank bottom, and dipping a work conveyed by a conveyor in the processing solution, thereby conducting electrodeposition coating or other surface treatment.
2. Statement of Related Art
In electrodeposition coating, an electrodeposition coating material dissolved or dispersed in water at a solid concentration of 8 to 20% is contained in an electrodeposition tank, metal works are dipped therein and a DC voltage at about 100 to 300V is applied between the work and the electrode in the tank thereby electrically depositing the coating material ingredient (solids) present in the form of ions in the solution on the surface of the work at an opposite polarity to form coating layers.
In a case where the coating material conditions and the current supply condition are kept constant, coating can be applied at an identical distribution of layer thickness uniformly as far as shaded portions of the work with good depositability. Particularly, since the cationic electrodeposition coating method using a work as the cathode has excellent anti-rust performance not leaching metals from the work, this method is adopted for primary coating of most of automobile bodies.
FIG. 3 shows an existent electrodeposition apparatus 41 comprising a navicular electrodeposition tank (processing solution tank) 42 for dipping a work W such as an automobile body to be conveyed by a conveyor (not illustrated) and a sub-tank 44 provided on one end of the tank 42 for recovery of bubbles or dusts suspended to the liquid surface by way of an overflow dam 43.
At the bottom and on the side in the electrodeposition tank 42, are arranged a plurality of spray nozzles 45 for discharging an electrodeposition solution recovered in the sub-tank 44 thereby stirring the inside of the electrodeposition tank 42. Electrodes (not illustrated) are provided at the inside of the tank for forming electric fields relative to the work W, and current supply bars (not illustrated) for maintaining the work W to a polarity opposite to that of the electrodes are arranged along the conveyor.
For the spray nozzles 45, nozzles 45T arranged on the side of the liquid surface of the electrodeposition tank 42 are arranged forward in the conveying direction of the conveyor, while nozzles 45B arranged at the bottom of the tank are directed backward in the conveying direction of the conveyor, such that the electrodeposition solution forms an agitating flow flowing in the directions opposite to each other between the side of the liquid surface and the side of the tank bottom.
With the constitution, the coating material ingredients are agitated with no precipitation in the electrodeposition tank 42 to form more uniform coating layers.
However, in a case where high quality electrodeposition coating layers are required, it has been found that dusts such as iron powders and paint wastes are adhered on the surface to cause coating failure referred to as seeds even when the flow of the agitating flow is formed.
In electrodeposition coating of automobile bodies, the seeds are observed particularly at those portions of large horizontal area such as a roof or an engine hood situated at the upper half of the body but they are not formed so much on a fender or a door.
As a result of experiments and studies made by the present inventors, this is considered to be attributable to that dusts, etc. tend to be placed on the portions of the large horizontal area, as well as that the agitating flow is formed in the forward direction of the conveyor on the side of the liquid surface in which the upper half of the automobile body is dipped and, accordingly, the relative speed between the automobile body and the deposition solution is low.
In view of the above, an electrodeposition tank as shown in FIG. 4 was proposed in which a sub-tank 52 is provided on the inlet of an electrodeposition tank 51, all the spray nozzles in the tank 51 are directed backward relative to the conveying direction of the conveyor from the liquid surface to the lower portion of the tank to flow the electrodeposition solution only in the direction from the exit to the inlet of the tank, so as to increase the relative speed of the electrodeposition solution to the work W to be conveyed from the inlet to the exit of the tank.
However, when the present inventors have analyzed the flow of the electrodeposition solution in the electrodeposition tank 51 described above, it has been found that the solution does not always form a uni-directional stream in the electrodeposition 51 but partially formed backflow/turbulence as shown in FIGS. 5(a) and (b).
That is, since the spray nozzles 53 are arranged only at the lower portion and on the side of the tank, while the electrodeposition solution flows from the exit to the inlet of the tank in the portions along the bottom and the side of the tank, but it causes backflow/turbulence near the central portion.
FIG. 5(a) shows the flow at the vertical cross section for the central portion of the electrodeposition tank along the conveying line of the conveyor, in which a flow from the exit to the inlet of the tank is formed along the bottom of the tank but includes a great deal of backflow and also some turbulence.
Further, FIG. 5(b) shows the flow on the liquid surface. While a flow from the exit to the inlet of the tank is formed along right and left sides, it can be seen that the backflow/turbulence are formed along the center line.
As described above, even when the spray nozzles 53 are arranged in the identical direction intending to form a flow in one direction in the electrodeposition tank 51, since backflow/turbulence are formed near the central portion in which the automobile body is conveyed, seeds tend to be formed when the relative speed is lowered by backflow and since dusts are scattered upward by the turbulence, they have an increased probability of adhesion, both of which cause coating failure.
Further, when the electrodeposition tank 42 shown in FIG. 3 has been analyzed in the same manner, it has been found that backflow/turbulence also occur therein.